In the United States, over two million women breastfeed their babies each year. World-wide, between 50 and 100 million breastfeed annually. In some countries, as many as 90% of the women breastfeed for longer than a year. Lactational amenorrhea (LAM) causes infertility. This period of infertility contributes to decreased birth rates for at least 50% of the 100 million women who deliver annually. LAM is an important factor in limiting population growth. The objective of this project is to define the hormone parameters associated with lactational amenorrhea with the purpose of facilitating family planning. We seek a method of delineating the boundaries between the infertile and fertile states.